The pulmonate snail Biomphalaria glabrata and a naturally-occurring parasite, the human blood fluke Schistosome mansoni, are being employed as a mode host-parasite system to study the immunologic parameters regulating the establishment of larval schistosome infections within genetically-defined snail stocks. For the coming year research efforts will be directed towards (1) identifying and isolating hemolymph (serum) components from PR Albino (schistosome-susceptible) and 10-R2 (schistosome-refractory) snails which are shared with larval S. mansoni, and determining whether such components can alter parasite-hemocyte interactions in vitro, (2) identifying exogenous materials of snail origin which are able to bind the surface tegument of S. mansoni sporocysts, and evaluating whether this material may be mediating parasite recognition (opsonizing substances) or nonrecognition ("masking" substances) by snail hemocytes, and (3) comparing, using antibody and lectin membrane probes, the molecular composition of the surface membranes from PR Albino and 10-R2 snail hemocytes in an attempt to distinguish and characterize snail-stock specific hemocyte subpopulations.